See that F is the force by 2 -Qs on q... each of which is equal to
F=Q∗q /(4Πϵ ∗L^2)
and are attractive..
And each of these F act on q at 90 degree.. so their resultant is F ' =
F*√2
towards the inside of the square .. along the diagonal..
And, the force on q by 4Q is
4Q∗q/(4Πϵ *2d^2)
= 2F.. along the diagonal but outside the square..
So the forces 2F and F√2 are opposite .. Hence the resultant force will be
2F−F√2 = (2-√2) F
Now put the value of F=Q∗q /(4Πϵ ∗L^2) to get the actual force..resultant.
4Πϵ can be written into a single constant K.